In a measurement instrument that measures a signal having a high frequency as in a millimeter-wave band, a waveguide having a low loss in the millimeter-wave band is used as an input or output transmission line in many cases. In such a measurement instrument, when characteristics of an integrated circuit (IC) are evaluated, it is necessary to connect a strip line (a microstrip line or a coplanar line) formed on a printed board on which an IC to be tested is mounted with a waveguide of the measurement instrument. However, the impedance of the strip line is generally about 50 to 100Ω, whereas the impedance of the waveguide is several hundreds of Ω. For this reason, it is not easy to achieve impedance matching.
As a technology of solving such a problem, there has been known a method of bringing a coupling ridge portion of a ridge waveguide into contact with a microstrip line as in Patent Document 1 or a method of vertically inserting a microstrip line from a side surface of a waveguide as in Patent Document 2.